Wireless communication base stations, networks, and other systems use power amplifiers, such as to transmit signals to cellular phones, computers, personal electronic assistants, and other devices. A power amplifier increases the average power of the transmitted wireless signal sufficiently to maintain a reliable communication link at any required distance. The transmitted signal's power varies, depending on both the modulation type and the data sequence being transmitted, resulting in the signal exhibiting random peaks and troughs over time in its instantaneous power. The complexity and cost of an amplifier is highly dependent on the maximum instantaneous power that it must accommodate. Consequently, base station providers and other electronics users seek ways to lower the instantaneous or “peak”-power requirements of the relevant system.
To reduce system peak-power requirements, a provider may simply limit the maximum amplifier output power by constraining or “clipping” the maximum magnitude of its output signal. Clipping the amplifier output effectively reduces the peak-power output requirement while still providing ordinary amplification for non-peak signals. Since the cost of a power amplifier rapidly increases as it is required to accommodate higher peak-power levels, clipping can significantly reduce system cost.
Clipping may be particularly attractive in applications in which occasional large peaks occur. For example, in wireless communications, a single amplifier often simultaneously amplifies signals for multiple channels. Occasionally, the multiple signals constructively combine to generate a relatively high peak. The amplifier must either fully amplify the peak, requiring an expensive high peak-power amplifier, or the output magnitude may be clipped to facilitate the use of a lower peak-power, less expensive amplifier.
In wireless communications and networking, however, clipping is unacceptable. Clipping induces spectral regrowth, creating spectral energy in potentially restricted spectral regions. The electromagnetic spectrum is a finite resource, and it is strictly apportioned by restrictions from various regulation agencies to minimize interference from competing users. The various spectrum users receive permission to transmit within certain bandwidths and are ordinarily prohibited from transmitting outside of the designated bandwidth. Even within the so-called “unlicensed bands”, strict FCC standards regulate spectral emissions to minimize interferences. Because spectral regrowth adds unacceptable frequency components to the signal, spectrum regulations do not permit clipping as a solution for high power amplifier requirements.